Based on channel information available at a transmitter, the transmission capacity of the channel can be increased by adapting link parameters like transmission power, modulation level, and channel coding. This is called link adaptation (LA) in general. If the transmission power is held constant while modulation and coding scheme is varied accordingly with the channel realization, then it is called as Adaptive Modulation and Coding (AMC). In the following, various combinations of the modulation and coding is called as Modulation and Coding Scheme (MCS). Each of the MCS is identified by an index and that is called MCS index. Similarly, a standard form of the information about the channel state is called Channel State Information (CSI). The entire system bandwidth is divided into smaller units for two different purposes: resource allocation and channel information feedback. With respect to resource allocation the system bandwidth is divided into smaller units called Resource Allocation Units (RAU). Similarly, for channel information reporting the same system bandwidth is divided in different granularity (finer or coarser) and it's called as Channel Information Reporting Unit (CIRU). The receiver reports a single CQI index for each of the CIRU. The granularity of CIRU may be greater or lesser than the granularity of RAU. If granularity of RAU is greater, then more than one RAU may constitute a CIRU. In which case, all the RAUs corresponding to that CIRU assume the MCS index value mapped from the corresponding channel quality index (CQI index). If the granularity of the RAU is smaller, then more than one CIRU may constitute a RAU. In this case, the RAU assumes the average value of MCS indices mapped from each of the CQI index corresponding to the constituent CIRUs. Eventually, each of the RAU has a MCS index associated to it which can be used by scheduler and eventually by the transmitter when transmitting on it.
Consider a system with N UEs. Let there be S number of CIRU and R number of RAU in the system. Then, let qn,s(m), m=1, 2 . . . ∞ up denote the CQI reported on the sth CIRU by nth receiver during mthsub-frame and qn,sεQ, where Q is the set of all possible CQI indices. Similarly, let ψn,s(m), m=1, 2 . . . . ∞ denote the MCS Index chosen for the transmission of the rth RAU to nth receiver during mth sub-frame and ψn,rεΨ. Ψ is the set of all possible MCS indices and L is the cardinality of Ψ which is actually the total number MCS indices available. Then, the mapping function f:Q→Ψ depends on the wireless technology that is used. While each CIRU is associated with qn,s, each RAU is associated with ψn,r. Thus, qn,s forms the input of the AMC scheme and ψn,r serve as the input to the scheduling technique.
At any point in time qn,s(m) can be at least h sub-frames old, where h is the CQI reporting delay. Let φ, φ=0, . . . , h−1 denotes the sub frames elapsed since the last CQI feedback. Then (m−φ) is the sub frame when the last feedback happened and qn,s(m−φ) is the most recent CQI Index available for the sth CIRU in nth receiver. Thus, for the sub-frames from m−φ to m−φ+h, ψn,r can be mapped from qn,s(m−φ). The following steps are involved in the normal link adaptation system. The FIG. 1 described the steps involved in the normal link adaptation system.
At step 102: [Sub-frame m, Transmitter]: Perform HARQ transmission. After forming a transmission unit whose size is determined by a MCS index used and a number resource allocation unit allocated. Based on the number of RAUs allocated during scheduling and the MCS Index chosen, an eNodeB can decide the amount of application data that can be transmitted in this transmission opportunity. The eNodeB combines all the allocated RAUs, adds Forward Error Correction (FEC) coding information and transmits it as Transport Blocks (TB). This FEC coding information used by the UE to do error detection and error correction in case of erroneous transmission. Either if there is no error or if the FEC coding is sufficient to do error correction the UE sends a HARQ ACK, else sends a HARQ NACK. When a NACK is received, the eNodeB increases the coding level and transmits the same TB, when the corresponding UE gets scheduling opportunity. This process is repeated either until the UEs receives the TB without error or if the process has been repeated a predetermined maximum number of times. There is an unavoidable delay between the instance at which the eNodeB transmits the TB and the instance at which it receives ACK/NACK from the UE. But the eNodeB can schedule the same UE using another instance HARQ. Each instance of HARQ is called HARQ process.
At step 104: [Sub-frame m, Receiver]: Perform HARQ reception to determine if a current transmission is HARQ success or failure.
At step 106: [Sub-frame m+b, Receiver]: Transmit ACK/NACK feedback to the transmitter.
At step 108: Determine whether the current subframe is a CQI feedback sub frame,
At step 110: If the current feedback CQI feedback then the CQI information is reported to the transmitter. This step does not happen every sub frame.
At step 112: [Sub-frame m+b+1, Transmitter]: The transmitter maps qn,s to ψn,s.
At step 114: [Sub-frame m+b+1, Transmitter]: The transmitter performs bandwidth scheduling which allocates each RA unit to a receiver n based on a scheduling criteria.